Bean leaf beetle populations in Iowa have changed dramatically during the last decade. Beginning in 1996, populations shot upward from a low of about 4 second generation beetles per 50 sweeps to a historical high in 2002 of 201 second generation beetles per 50 sweeps (see graph below). This population increase was most likely due to favorable winter conditions, such as mild temperatures or insulating snow cover, and an increase in soybean acres that were planted earlier in the spring.
|Bean leaf beetles on early-emerging soybean.
In 2003, bean leaf beetle populations took a nosedive and returned to densities similar to 1996. There were two primary reasons for this population crash. First, nearly three million soybean acres were sprayed with insecticides during late July and August 2003 for control of soybean aphids. This time period would have coincided with the beginning of the second generation bean leaf beetles. Second,
August 2003 was the driest recorded August in Iowa. Throughout much of the state less than one inch of rain fell--and rain is critical for moistening the soil so the adult beetles can dig their way to the surface. Many beetles remained entombed in the dry soil, but those that did emerge were hit with an insecticide.
This population reduction is, of course, good news for soybean growers. However, we had a fairly mild winter in Iowa so overwintering survival of the beetles was higher than during the last two winters. Estimated mortality is shown in the map on the back. This information cannot be used to make management recommendations, but it does provide an indication of insect potential. These insects will be back in soybean fields this spring, but overwintering populations are expected to be low.
Remember, the earliest emerging soybean fields typically attract the largest populations of bean leaf beetles. These fields should be closely scouted.
More information will be provided next week on scouting and management recommendations.
Bean leaf beetle second generation populations, Ames, Iowa, 1989-2004.
Estimated bean leaf beetle winter mortality, 2005.
This article originally appeared on pages 55-56 of the IC-494 (7) -- April 25, 2005 issue.